Method for producing propylene-based resin microporous film and propylene-based resin microporous film

Abstract

Provided is a method for producing a propylene-based resin microporous film capable of forming a high-performance lithium ion battery. The method for producing a propylene-based resin microporous film includes an extrusion step of melt-kneading a propylene-based resin in an extruder, and extruding the resin to obtain a propylene-based resin film; a first stretching step of uniaxially stretching the propylene-based resin film at a surface temperature of −20 to 100° C.; second stretching step of repeating, a plurality of times, a stretching basic step in which the propylene-based resin film after the first stretching step is uniaxially stretched at a surface temperature that is equal to or lower than a temperature lower than the melting point of the propylene-based resin by 10 to 100° C., wherein between successive stretching basic steps, the stretching ratio in the forward-side stretching basic step is adjusted so as to be lower than that of the backward-side stretching basic step; and an annealing step of annealing the propylene-based resin film after the second stretching step.

The invention claimed is: 
     
       1. A method for producing a propylene-based resin microporous film, comprising:
 an extrusion step of melt-kneading a propylene-based resin at a temperature that is equal to or higher than a temperature higher than a melting point of the propylene-based resin by 20° C. and equal to or lower than a temperature higher than the melting point of the propylene-based resin by 100° C. in an extruder, and extruding the propylene-based resin through a T die attached to the tip of the extruder to obtain a propylene-based resin film; 
 a first stretching step of uniaxially stretching the propylene-based resin film at a surface temperature of −20° C. to 100° C. and a total stretching ratio of 1.05 times to 1.6 times; 
 a second stretching step of a stretching basic step in which the propylene-based resin film after the first stretching step is uniaxially stretched in the stretching direction of the propylene-based resin film at a surface temperature that is higher than the surface temperature of the propylene-based resin film in the first stretching step and equal to or lower than a temperature lower than the melting point of the propylene-based resin by 10° C. to 100° C., wherein the total stretching ratio is 1.05 times to 3 times and between the successive stretching basic steps, the stretching ratio in the forward-side stretching basic step is adjusted so as to be lower than the stretching ratio in the backward-side stretching basic step, 
 wherein the stretching basic step is repeated a plurality of times; and 
 an annealing step of annealing the propylene-based resin film after the second stretching step at a surface temperature that is equal to or higher than the surface temperature of the propylene-based resin film in the second stretching step and equal to or lower than a temperature lower than the melting point of the propylene-based resin by 10° C. 
 
     
     
       2. The method for producing a propylene-based resin microporous film according to  claim 1 , wherein the propylene-based resin has a weight average molecular weight of 250,000 to 500,000, a molecular weight distribution of 7.5 to 12.0, and a melting point of 160° C. to 170° C. 
     
     
       3. The method for producing a propylene-based resin microporous film according to  claim 1 , comprising, before the first stretching step is performed, an aging step of aging the propylene-based resin film obtained in the extrusion step at a temperature that is equal to or higher than a temperature lower than a melting point of the propylene-based resin by 30° C. and equal to or lower than a temperature lower than the melting point of the propylene-based resin by 1° C. for 1 minute or more.